My readers know me as The Wildlife Ho-tographer, using Canon equipment for over 30 years. I have been an EOS-1 user since 1989 when the first model came out and currently use the EOS-1D X Mark II as my main body.
You can read my post on the upcoming EOS-1D X Mark III here for possible camera specifications and follow my travels on Facebook, Twitter, Instagram and this blog plus my website.
Canon announced the EOS R system featuring the newly designed RF mount on September 5, 2018. You can download the whitepaper on this new body here. I was invited by Canon to a hands-on demonstration of the production camera on September 9. You can read my earlier post in October 2018, explaining why a long time user like myself will not buy the current EOS-R mirrorless camera.
Now it appears Canon may have listened to users like myself and is working on a pro version of the EOS-R mirrorless camera in an EOS-1 body and use both the EF and RF lenses without the need for an adapter.
As I mentioned in my 2018 post, there is no technical reason why Canon cannot make an EOS-1D mirrorless camera. They chose to go with the less weight solution and came up with an also-ran camera called EOS-R plus a new, expensive RF line of lenses.
In order for the EOS-1D camera body to accept the RF and EF lenses, Canon engineers will have to compensate for the different sensor-to-flange distance (20mm vs 44mm) and electrical connectors (12 vs 8) between the two mounts.
These difference can be overcome if Canon decides to listen to their professional and serious user community and remedy their mistake once and for all. The EOS-1D X Mark III will be announced in Q1, 2020. After that, Canon can work on the pro EOS-1D R mirrorless model and have a camera ready perhaps in late 2020 or early 2021. Keep checking back for the latest information and development.